Like a call made using a mobile telephone, with speech communication in a mobile communication system, currently, communication using a monaural scheme (monaural communication) is a major stream. However, hereafter, like a fourth generation mobile communication system, if a transmission rate becomes a still higher bit rate, it is possible to ensure a bandwidth for transmitting a plurality of channels, and therefore it is expected that communication using a stereo scheme (stereo communication) will be also widespread in speech communication.
For example, when it is considered that the current situation where the number of users increases who enjoy stereo music by recording music in a mobile audio player provided with a HDD (hard disc) and attaching earphones or headphones for stereo to the player, in the future, it is expected that mobile telephones and music players will be linked together and a life style will be prevalent where speech communication is carried out using a stereo scheme utilizing equipments such as earphones and headphones for stereo. Further, in an environment such as Video conference that has recently become widespread, in order to enable conversation having high-fidelity, it is expected that stereo communication is performed.
On the other hand, in a mobile communication system and wired communication system, in order to reduce load of the system, it is typical to achieve a low bit rate of transmission information by encoding speech signals to be transmitted in advance. As a result, recently, a technique for encoding stereo speech signals attracts attention. For example, there is a coding technique for increasing the coding efficiency for encoding predictive residual signals to which weight of CELP coding for stereo speech signals is assigned, using cross-channel prediction (refer to non-patent document 1).
Furthermore, even if stereo communication becomes widespread, it is also expected that monaural communication will still be carried out. This is because monaural communication is carried out at a low bit rate and its communication cost is expected to be reduced, and moreover, a mobile telephone supporting only monaural communication has a small circuit and is inexpensive, so that users who do not want high quality speech communication will prefer to purchase a mobile telephone supporting only monaural communication. Therefore, there will be mobile telephones supporting stereo communication and mobile telephones supporting monaural communication in one communication system, and the communication system needs to support both stereo communication and monaural communication. Moreover, in a mobile communication system, communication data is exchanged using radio signals, and therefore, a part of the communication data may be lost depending on a channel environment. Therefore, it will be very useful if a mobile telephone has a function of restoring original communication data from the rest of received data, even when the part of the communication data is lost.
There is scalable coding formed with a stereo signal and a monaural signal as a function of supporting both stereo communication and monaural communication and restoring original communication data from the rest of received data, even when the part of the communication data is lost. As an example of a scalable coding apparatus having this function, there is an apparatus disclosed in Non-Patent Document 2.
Non-Patent Document 1: Ramprashad S. A., “Stereophonic CELP coding using cross channel prediction”, Proc. IEEE Workshop on Speech Coding, Pages: 136 to 138, (17 to 20 Sep. 2000)
Non-Patent Document 2: ISO/IEC 14496-3:1999 (B.14 Scalable AAC with core coder)